Generally, solar light reaches the earth while electric and magnetic fields fluctuate, like a wave, and has heat, and thus, is called ‘thermal rays’. Light is classified into X-rays, ultraviolet rays, visible light, infrared rays, etc., depending on particles, waves and frequencies of vibration, and X-rays are permeable, ultraviolet rays and visible light are reflexible, and infrared rays are absorbable. Of infrared rays, far-infrared rays have a longer wavelength than visible light, and thus, are regarded as a sort of electromagnetic wave having a frequency of 1.5-1000 μm. Far-infrared rays, which are light energy having a slightly longer wavelength of 5-25 μm among infrared rays, have been used for various purposes from far-infrared sauna to home appliances, construction materials, general living goods, etc., because they have been determined to be beneficial to human beings in recent years.
To manifest biofunctions, far-infrared emissivity should be not less than 0.9. Further, antibiotic activity is assayed by measuring the rate of bacterial reduction using Escherichia coli and Pseudomonas aeruginosa. In particular, in the case of a precoated metal sheet for use in home appliances, it should have high processability and gloss.
As a conventional technique concerned with a paint composition having far-infrared emissivity, Korean Patent Application No. 1997-18446 discloses the use of a material emitting far-infrared rays by impregnating zeolite with Zn and Ag. However, the material used is expensive, thus negating economic benefits, and also, the far-infrared emissivity is as low as 0.90.
On the other hand, the paint for precoated metal sheets used for construction materials need be hardly processed; it has larger amounts of fillers. Of the fillers, silica is added in a relatively large amount of 10-15 parts by weight, which has been used to decrease the alkalinity of conventional far-infrared emitting powder. However, in the case of the paint for precoated metal sheets used for home appliances, silica undesirably acts to retard the processability.
As the far-infrared emitting material, jade and elvan (Korean Patent Application Nos. 1988-0001616 and 1995-0026761) are representatively known. In Japanese Patent Laid-open Publication No. 2002-53812, a paint including apatite, torumaline and charcoal to emit far-infrared rays and anions is disclosed. However, since the materials mentioned above are expensive and have a large particle size, they are unsuitable for use in precoated metal sheets.
Japanese Patent Laid-open Publication No. 2003-171604 discloses an antibacterial coating material comprising a silicone resin and antibacterial photocatalytic powder. However, such a paint is unsuitable for precoated metal sheets for home appliances due to its low processability.
Korean Patent Application No. 1998-709300 discloses an anti-fouling silicone emulsion coating composition. However, it is difficult to use such a composition in the fields requiring processability. In addition, since silicone functions as a matting agent, the above composition is unsuitable for use in highly glossy products.
Japanese Patent Application No. 1999-319848 discloses a method of forming a photocatalytic layer which has excellent alkali resistance and water resistance using a silicone modified acryl resin. However, the use of silicone modified acryl resin results in decreased processability and gloss, and the above resin is undesirable as a resin component for use in precoated metal sheets.
U.S. Pat. No. 6,376,559 discloses a method of preparing a coating composition comprising a dispersing component having an inorganic silica sol dispersed in a solvent. However, the solution system used in the above invention is unsuitable for a coating process.
Korean Patent Application No. 1997-702389 discloses a thermosetting composition having high anti-fouling characteristics, weather resistance and chemical resistance using a ceramic component, a coating finish method and a coated article, and Korean Patent Application No. 2001-43214 discloses a urethane based coating resin composition having high gloss, weather resistance, hardness and flexibility, and a curing coating composition including the same. In addition, U.S. Pat. No. 6,022,919 discloses a coating composition comprising a resin produced by copolymerizing monomers containing methacrylate ester, OH group, COOH group, styrene and acrylonitrile. However, the coating compositions as mentioned above do not exhibit far-infrared emissivity (emission), antibiotic activity, solvent resistance, gloss, processability and storage stability to the extent of being suitable for use in precoated metal sheets for home appliances.